Methods and circuits for measuring mutual and self capacitance

Abstract

An embodiment of a capacitance measurement circuit may include multiple switches, a first node coupled with a first electrode and coupled with at least a first switch of the multiple switches, and a second node coupled with a second electrode and coupled with at least a second switch of the multiple switches, where the multiple switches are configured to reduce an influence of a self-capacitance of the first electrode and a self-capacitance of the second electrode on an output signal during measurement of a mutual capacitance between the first electrode and the second electrode, and where the multiple switches are configured to reduce an influence of the mutual capacitance on the output signal during measurement of at least one of the self-capacitance of the first electrode and the self-capacitance of the second electrode.

What is claimed is: 
     
       1. A capacitance measurement circuit comprising:
 a capacitance to current conversion circuit configured to operate in a self capacitance mode in a first operational state and configured to operate in a mutual capacitance mode in a second operational state; and 
 a current to code conversion circuit configured to output a digital value representative of a current generated by the capacitance to current conversion circuit; wherein:
 when the capacitance to current conversion circuit is configured to operate in the self capacitance mode, the capacitance to current conversion circuit comprises a sense capacitor between a first voltage potential and a plurality of switches, wherein the plurality of switches are configured to couple the sense capacitor alternately to a second voltage potential and to the current to code conversion circuit; and 
 when the capacitance to current conversion circuit is configured to operate in the mutual capacitance mode, the capacitance to current conversion circuit comprises a sense capacitor between a first electrode and a second electrode, the first electrode is coupled to an alternating voltage signal, and the second electrode is coupled to the current to code conversion circuit. 
 
 
     
     
       2. The capacitance measurement circuit of  claim 1 , wherein the current to code conversion circuit comprises;
 an analog-to-digital converter (ADC); and 
 an integration capacitor coupled to the output of the capacitance to current conversion circuit and an input of the analog to digital converter at a first integration capacitor electrode and coupled to a fixed voltage potential at a second integration capacitor electrode. 
 
     
     
       3. The capacitance measurement circuit of  claim 1 , wherein the current to code conversion circuit comprises;
 a comparator; 
 an integration capacitor coupled to the output of the capacitance to current conversion circuit and an input of the comparator at a first integration capacitor electrode and coupled to a fixed voltage potential at a second integration capacitor electrode; and 
 a time measurement circuit coupled to the output of the comparator. 
 
     
     
       4. The capacitance measurement circuit of  claim 1 , wherein the current to code conversion circuit comprises:
 a transimpedance amplifier (TIA) coupled to the output of the capacitance to current conversion circuit and to an analog-to-digital converter (ADC); and 
 an integration capacitor coupled between an input and an output of the TIA. 
 
     
     
       5. The capacitance measurement circuit of  claim 1 , wherein the capacitance measurement circuit is configured to switch between the first operational state and the second operation state and the capacitance to current conversion circuit is configured for either self capacitance measurement or mutual capacitance measurement. 
     
     
       6. A capacitance sensing system comprising:
 a first plurality of electrodes disposed along a first axis; 
 a second plurality of electrodes disposed along a second axis; 
 a capacitance to current conversion circuit coupled to the first plurality of electrodes and the second plurality of electrodes, the capacitance to current conversion circuit configured to operate in a self capacitance mode in a first operational state and configured to operate in a mutual capacitance mode in a second operational state; and 
 a current to code conversion circuit, wherein the current to code conversion circuit is configured to output a digital value representative of a current generated by the capacitance to current conversion circuit; wherein:
 when the capacitance to current conversion circuit is configured to operate in the self capacitance mode, the capacitance to current conversion circuit comprises a sense capacitor between a first voltage potential and a plurality of switches, wherein the plurality of switches are configured to couple the sense capacitor alternately to a second voltage potential and to the current to code conversion circuit; and 
 when the capacitance to current conversion circuit is configured to operate in the mutual capacitance mode, the capacitance to current conversion circuit comprises a sense capacitor between a first electrode and a second electrode, the first electrode is coupled to an alternating voltage signal, and the second electrode is coupled to the current to code conversion circuit. 
 
 
     
     
       7. The capacitance sensing system of  claim 6 , wherein the capacitance to current conversion circuit, when configured to operate in the self capacitance mode, is coupled to at least one of the first or second pluralities of electrodes and is configured to convert the capacitance of the at least one of the first or second pluralities of electrodes to a current value. 
     
     
       8. The capacitance sensing system of  claim 6 , wherein the capacitance to current conversion circuit, when configure to operate in the mutual capacitance mode, is coupled to at least one of the first plurality of electrodes and at least one of the second plurality of electrodes and is configured to convert the mutual capacitance between the at least one of the first plurality of electrodes and the at least one of the second plurality of electrodes to a current value. 
     
     
       9. The capacitance sensing system of  claim 6 , wherein the capacitance to current conversion circuit, when configured to operate in the mutual capacitance mode, comprises:
 a mutual capacitance between at least one of the first plurality of electrodes and at least one of the second plurality of electrodes; 
 a first plurality of switches configured to couple the at least one of the first plurality of electrodes to an output of a voltage buffer and to couple the at least one of the second plurality of electrodes to an input of the voltage buffer and to the current to code converter in non-overlapping phases; and 
 a second plurality of switches configured to couple the at least one of the first plurality of electrodes to a fixed voltage potential and to couple the at least one of the second plurality of electrodes to the input of the voltage buffer and to the current to code converter in non-overlapping phases. 
 
     
     
       10. The capacitance sensing system of  claim 6 , wherein the first and second pluralities of electrodes are a sensitive portion of a touchscreen. 
     
     
       11. The capacitance sensing system of  claim 6 , wherein the capacitance sensing system is configured to detect a single conductive object in the first operational state and configured to detect multiple conductive objects in the second operational state. 
     
     
       12. A method of measuring capacitance, comprising:
 at a capacitance sensing system having a first plurality of electrodes disposed along a first axis, a second plurality of electrodes disposed along a second axis, a capacitance to current conversion circuit coupled to the first plurality of electrodes and the second plurality of electrodes, and a current to code conversion circuit:
 in a first operational state, configuring the capacitance to current conversion circuit to convert a self capacitance to a first current, wherein the current conversion circuit comprises a sense capacitor between a first voltage potential and a plurality of switches, and wherein the plurality of switches are configured to couple the sense capacitor alternately to a second voltage potential and to the current to code conversion circuit; 
 converting a self capacitance to the first current; 
 converting the first current to a digital value representative of the self capacitance; 
 in a second operational state, configuring the capacitance to current conversion circuit to convert a mutual capacitance to a second current, wherein the capacitance to current conversion circuit comprises a sense capacitor between a first electrode and a second electrode, the first electrode is coupled to an alternating voltage signal, and the second electrode is coupled to the current to code conversion circuit; 
 
 converting a mutual capacitance to the second current; and 
 converting the second current to a digital value representative of the mutual capacitance. 
 
     
     
       13. The method of  claim 12 , wherein converting the self capacitance to the first current comprises:
 coupling a capacitor to a first voltage potential in a first non-overlapping phase; and 
 coupling a capacitor to a second voltage potential in a second non-overlapping phase. 
 
     
     
       14. The method of  claim 12 , wherein converting the mutual capacitance to the first current comprises:
 coupling a first electrode to an alternative voltage signal; and 
 coupling a second electrode to an integration capacitor. 
 
     
     
       15. The method of  claim 12 , wherein converting the mutual capacitance to the first current comprises:
 accumulating charge on an integration capacitor; 
 generating a voltage potential across the integration capacitor; and 
 converting the generated voltage potential across the integration capacitor to a digital value. 
 
     
     
       16. The method of  claim 12 , further comprising detecting a first conductive object in the first operational state and detecting a second conductive object in the second operational state.